Recently, in order to facilitate operations of referring to a great amount of accumulated data of electronic documents, electronic mails, logs of observation data, or the like, it is considered desirable that the data be stored in a medium which is accessible at any time from the viewpoint of improvement and security of business processes.
In order to store data as above, a highly-reliable large-capacity storage device which can store data for a long time is needed. In addition, it is desirable that the storage device be relatively inexpensive. It is generally known that the RAID (Redundant Arrays of Inexpensive Disks) devices, each of which has a plurality of disks realizing a redundant structure, satisfy the above requirements. The redundant structure is a structure in preparation for an emergency. For example, in the redundant structure, a backup disk may be provided in addition to a disk which is mainly used, or a plurality of disks are provided and the used disk is switched among the plurality of disks. Further, virtual storage systems can be realized by connecting a plurality of modules having RAID devices, and are widely used.
Since the RAID devices redundantly store data, the RAID devices enable recovery of data even when a trouble occurs. However, when a trouble which goes beyond the redundancy of a RAID device occurs, data may not be recovered and is lost. Therefore, in order to further improve the reliability of a large-scale system, a hierarchic redundant structure such as a data redundant structure realized by interconnecting a plurality of RAID devices has been proposed (for example, as disclosed in Japanese Laid-open Patent Publication No. 2000-076207).
Nevertheless, when a trouble which is irrecoverable by the redundant structure in a node (module) is caused by a readout failure occurring in a RAID device, an operation for recovery of the entire RAID device is necessary according to the technique disclosed in Japanese Laid-open Patent Publication No. 2000-076207.
Further, when troubles concurrently occur in a plurality of nodes, are irrecoverable in the respective nodes, and go beyond the redundancy across the nodes, the recovery from the troubles is impossible.